An in-flight operation involves one or more aircraft performing a maneuver or exercise. One example of an in-flight operation is in-flight refueling. In-flight refueling is a strategic method to extend the range and effectiveness of aircraft when it is not feasible or desirable to land for ground refueling. An in-flight refueling operation typically involves a supplying aircraft (e.g., a tanker) and a receiving aircraft (e.g., a fighter, bomber, transport, or command and control aircraft). The supplying aircraft holds a substantially steady flight position while the receiving aircraft maneuvers into a refueling envelope behind and below the supplying aircraft. In one refueling method, the supplying aircraft lowers a boom with an attached refueling nozzle that is maneuvered by an operator to make contact with a refueling receptacle on the receiving aircraft. In a second refueling method, the supplying aircraft releases a refueling hose with an attached drogue within an appropriate range of the receiving aircraft. The receiving aircraft must maneuver so that a probe extending from the receiving aircraft makes contact with the drogue from the supplying aircraft. Present in-flight refueling systems commonly use a camera vision system (e.g., a remote aerial refueling operator (RARO) system as used on the Boeing KDC-10 and KC-767A/J) in which a video camera is pointed at the refueling envelope and a refueling operator views a display to perform operations. A computer system is also commonly used to process and enhance the display images. Sufficient illumination is needed for the video camera so that an operator viewing a display has good visibility of the refueling operation.
One challenge of in-flight refueling is providing sufficient illumination for the RARO system when performing night time in-flight refueling. It is desirable to minimize the use of visible light (e.g., radiation in the electromagnetic spectrum visible to the naked human eye) during a night time in-flight refueling operation. One reason is because it is desired that an observer's attention not be drawn to the refueling operation. For example, an observer on the ground may perceive the visible light on a supplying aircraft during night time at a far distance when the atmosphere is substantially free of cloud cover, fog, dust, or other weather conditions which may obstruct visible light. In another situation, the observer may be in another aircraft instead of on the ground. Thus, there is a need for an illumination system that is imperceptible to the naked eye such that an observer on the ground or in an aircraft would not perceive the illumination being used during the in-flight refueling operation at night time and under substantially clear weather conditions.
A second reason why it is desirable to minimize the use of visible light is because the visible light interferes with night vision equipment (e.g., night vision goggles or night vision imaging systems). During night time operations, pilots wear night vision goggles (NVG). Night vision imaging systems (NVIS) may also be used onboard an aircraft. Visible light interferes with night vision equipment by causing “blooming”, loss of sensitivity, and is also destructive to the night vision equipment by over saturating the sensors. Even if night vision goggles are not used, the visible light can “blind” a pilot who looks directly into the light. Because the use of visible light is minimized or eliminated in night time in-flight refueling, there may be insufficient illumination such that the RARO cannot generate a practical image for use by the operator. Thus, there is a need for an illumination system for in-flight refueling that provides sufficient illumination for a camera vision system such as a RARO, and is compatible with night vision equipment.
Another challenge for night time in-flight refueling is to provide illumination that is uniformly incident on the receiving aircraft. Non-uniform illumination can cause glint, shadows, blooming, and glare when viewing the receiving aircraft through the camera and display system. Non-uniform illumination also makes it more difficult for computer systems to enhance images. For example, image enhancement algorithms that improve contrast may bring out detail in a darker part of an image, but may saturate and wash out the brighter parts of the image causing other detail to be lost. Adding to the challenge is the fact that a receiving aircraft comes in different shapes and sizes. When in the refueling envelope, providing uniform illumination incident on a fighter aircraft such as an F-16 may be substantially different than providing uniform illumination for a large command-and-control aircraft such as an E-3 AWACS. Furthermore, not all receiving aircraft have the same location for their refueling receptacles. Some refueling receptacles may be on the centerline of the main fuselage, some are off the centerline (e.g., F-15), and some may be in the nose (e.g., A-10 Warthog). Thus, there is a need for a system that provides illumination that is also configurable to provide uniform lighting over the receiving aircraft and around the refueling receptacle for different types of receiving aircraft.
Although solutions have been offered for illuminating in-flight refueling operations, many challenges still exist. As previously mentioned, radiation in the electromagnetic spectrum visible to the human eye is detectable, causes blooming and may be destructive to night vision equipment. Thus, visible light is not an effective solution. Illumination in the infrared (IR) or near-infrared electromagnetic radiation spectrum has been proposed as a solution that is undetectable to the naked eye. However, it is possible that aircraft or ground forces, also equipped with night vision equipment, may detect the infrared  light making the supplying or receiving aircraft a vulnerable target, and thus, compromise the mission. In addition, because night vision goggles are especially sensitive to IR or near-IR radiation, persons wearing night vision goggles may also be “blinded” by too much illumination in the same manner when a person is blinded by headlights of another vehicle.
Therefore, there exists a need for a system that provides illumination that is imperceptible to the naked eye and night vision equipment for in-flight operations such as in-flight refueling. The illumination of the system also needs to be compatible with night vision equipment, and the system should be configurable to uniformly illuminate different types of receiving aircraft for in-flight refueling.